Shielding Guidelines for Diagnostic X-ray Installation Flashcards
Role of Radiation Safety officer
- acts as an advisor on all radiation protection aspects during installation, initial construction and during subsequent operations
- ensure that the installation complies with all regulatory requirements
What types of radiation can go towards a primary barrier?
primary beam can be aimed at primary barrier
What types of radiation can go towards a secondary barrier?
only scattered and leakage radiation can go towards the secondary barrier
A floor plan must be prepared, clearly indicating which components?
- The dimensions and shape of the room where the X-ray equipment is operated and the physical orientation of the room (a mark indicating North).
- The location where the X-ray equipment is planned to be placed and the range of movement of the X-ray tubes.
- The location of the control booth, if applicable.
- The location, use, occupancy level and accessibility of adjacent rooms, as well as rooms above and below the facility.
- The designation of the adjacent rooms, whether to be designated as a controlled or uncontrolled area.
- The location where image processing is performed, i.e., location of darkrooms, film storage area, computer workstations.
- The position of all windows, doors, etc., that may affect radiation protection requirements.
- The planned and existing materials used to construct the walls, floor, ceiling, and the control booth, and their thicknesses including additional materials currently being used, or planned for use, as radiation shielding barriers.
- The application of the protective barriers. Will the intervening shield between the X-ray tube and the occupied area act as a primary or as a secondary protective barrier, i.e., will the barrier be required to attenuate the direct X-ray beam or stray radiation only?
Common Barrier Materials
Lead, Concrete, Gypsum Wallboard
Primary Barrier Equation
K = Pd^2/WUT
What does the P represent in the primary barrier equation?
Maximum permissible weekly exposure expressed in R/week
What does the d represent in the primary barrier equation?
distance in metres from the target to the primary area
What does the W represent in the primary barrier equation?
Workload in mA-min/week
What does the U represent in the primary barrier equation?
Use factor
What does the T represent in the primary barrier equation?
Occupancy factor
What does the K represent in the primary barrier equation?
Exposure per unit workload at unit distance, in R/ma-min at 1 metre
Assumptions made in shielding calculations
- The attenuation of the radiation beam by the patient is neglected.
- The incidence of the radiation beam is always perpendicular to the barrier being evaluated.
- The calculation does not take into account the presence of materials in the path of the radiation other than the specified shielding material.
- The leakage radiation from the X-ray equipment is assumed to be an air kerma of 0.876 mGy/h.
- The minimum distance to the occupied area from a shielded wall is assumed to be 0.3 m (30 cm)
Workload
- The workload is a measure of the operational time or the amount of use of the X-ray equipment
- A workload distribution indicates the workload across a range of operating voltages. The workload and workload spectrum can be determined by recording the operating voltage and current-time product of each irradiation taken in each X-ray suite over a set period of time (i.e., week).
- If actual workload values are not available, Table 5 presents estimated total workloads for various medical X-ray facilities
Occupancy factor
The Occupancy factor is the fraction of time that the area under consideration is occupied by the individual (employee or public) who spends the most time at that location while the X-ray equipment is operating.
Use factor
The use factor, is the fraction of the workload during which the X-ray beam is pointed in the direction under consideration.
For a controlled area what is the P value?
0.04 R/week
For an uncontrolled area what is the P value?
0.002 R/week
Determine the thickness of the primary barrier required to protect a CONTROLLED AREA that is 3 METRES from the target of a 150 kVp diagnostic unit having a weekly WORKLOAD of 2000 mA-min. The wall has a USE FACTOR of 1. On the other side of the wall is an ADJACENT X-RAY ROOM.
P = 0.04 R/week
d = 3 metres
W = 2000 mA - min
U = 1
T = 1
K = (0.04 x 3^2)/(2000 x 1 x 1)
K = (0.04 x 9)/2000
K = 0.36/2000
K = 0.00018
Using the chart the thickness of the primary barrier should be equivalent to 2.65 mm of lead OR 23.5 cm of concrete
